This invention relates to a multiplex transmission system based on a multiplex transmission system such as a CSMA/CD system applied to signal transmission in a vehicle or, more particularly, to a transmission system using a decentralized type multiplex communication network.
With the development of vehicle electronization, the problem of increase in the amount of wiring (wire harness) for connecting electronic parts becomes stringent. Multiplex communication is noticeable as a means for solving this problem particularly in the field of motor vehicles. A multiplex communication system is based on a serial transmission process in which a plurality of groups of data items are sent in a time-sharing multiplex manner.
In the field of motor vehicles, network forms of this multiplex communication are grouped into a complete multiplex type and a partially multiplex type or into a centralized type and a decentralized type. The partially multiplex type consists of a mixture of non-multiplex sections and multiplex sections. In multiplex sections, switches and loads distanced from each other are connected by multiplex transmission units. It is said that in this type of network the number of wires is increased although the total length of wires is reduced because separate wirings are required for connections between units, switches and loads. In a centralized type network, a plurality of slave transmission units are connected to one master transmission unit. However, it is said that the centralized type enables a reduction in the diameter of wires but is disadvantageous in that breakdown of the master directly results in system down and that the degree of design freedom is low.
On the other hand, the decentralized type is spotlighted though disadvantageous in terms of cost, because it enables a remarkable reduction in the wire diameter, is improved in reliability with respect to partial system down and has high flexibility with respect to design changes. (An example of this type of network is disclosed in Japanese Patent Laid-Open No. 62-4658.) For decentralized multiplex communication systems, a CSMA/CD method is adopted, as proposed in SAE (Society of Automotive Engineers) standardization. In this type of transmission system, a data frame having a destination address is transmitted from one of multiplex nodes, and the destination multiplex node designated by this destination address returns a reception confirmation signal (ACK) after the reception of the frame if it has correctly received the transmitted data from the transmission path. There has also been; by the Assignee of this invention proposed a PALNET (protocol for automobile local area network) system developed from the CSMA/CD system, as described in Japanese Patent Application Nos. 63-273305, 63-273306, and 63-273307. In this system, ACK signals are returned from all active nodes each time of signal reception; if sixteen nodes are connected, sixteen ACK signals are supplied to the transmission path. The destination node is discriminated with an identifier (ID). An ACK table is updated according to consistency/inconsistency between statuses before and after the transmission.
There is a possibility of occurrence of various kinds of transmission errors including errors relating to exchange between actuators, and parity errors. For simplifying a vehicle transmission system by utilizing multiplex communication, it is primarily important to consider the reliability of the system. In this case, the reliability is improved not by complete elimination of errors but accurate detection thereof and restoration from error states. In multiplex communication systems for motor vehicles, there is a possibility of instantaneous (intermittent) malfunctions due to electromagnetic spikes as well as solid (fixed) malfunctions including failures of electrical device elements and signals lines. In fact, instantaneous malfunctions occur more frequently.
According to the proposed Palnet system, if it is determined that a malfunction has occurred during reception or transmission, re-transmission sequence is repeated certain times (e.g., three times). If the number of re-transmission times exceeds the predetermined number, it is determined that the designated node has not existed or the system malfunctions have occurred.
However, there is no definite law which administrates instantaneous malfunctions and the number of times of repetition of the re-transmission sequence. In particular, during use in a motor vehicle, it is difficult to predict how long does an unstable transmission state continue. In other words, if the number of re-transmission times is excessively increased to cope with instantaneous malfunctions, the time taken to detect a solid failure is increased. If the number of re-transmission times is reduced, an instantaneous malfunction may be erroneously discriminated as a node malfunction.
In a motor vehicle control system in which a plurality of multiplex nodes are connected, possible errors are not limited to communication errors. For example, there is a possibility of inconsistency between the states of an input device element such as a switch and an output device element such as a display lamp, owing to a malfunction of the switch. For example, while the switch is turned on, the corresponding lamp is not lighted. It is difficult to detect such an inconsistency as a communication error. It is therefore important to solve the inconsistent state instead of detecting the same.